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In that case, the required digester volume is given by:

In determining the degree of thickening that can be accomplished during digestion it is often desirable to know the suspended solids concentration on a TSS basis. The TSS is just the sum of the VSS and the FSS, XM F:

Although some FSS are solubilized during digestion, as discussed in Section 12.1.1, many designers assume that they remain unchanged, thereby giving a conservative estimate of the TSS concentration. In that case, the FSS act like inert suspended solids, so that;

Xm.H

Thus, when solids are expressed on a TSS basis, the reactor volume is given by:

T-ec

The oxygen requirement (RO) can be calculated on the basis of the estimated VSS destruction efficiency:

The value of ¡o.xm.v depends on whether nitrification is occurring, as expressed in Table 12.1. Once RO is known, the air flow rate or power requirement for oxygen transfer can be estimated as was done in Section 10.2.5. That can then be compared to the power required for mixing as given by Eq. 12.10 or other appropriate information.

Alkalinity destruction can be estimated by recognizing that approximately 0.44 g of alkalinity (expressed as CaCO,) will be used for each g VSS destroyed if the released ammonia-N is nitrified. This can be compared with the available alkalinity to determine whether pH control will be needed. As discussed previously, a residual alkalinity of around 50 mg/L as CaCO, is required to maintain a stable pH near neutrality.

The following example illustrates the use of empirical correlations to accomplish a preliminary design.

### Example 12.3.2.1

A preliminary design is needed for a conventional aerobic digester using intermittent feeding with in-basin thickening for stabilization of waste biomass from an aerobic biological wastewater treatment system. At least 40% VSS destruction is desired year round. The waste biomass concentration is 8,000 mg/L as TSS and the flow rale is 500 m'/day. The solids are 75% volatile, but little else is known about their characteristics. The alkalinity of the carriage water is 150 nig/ L as CaCOt. The lowest operating temperature of the digester is expected to be 10°C based on similar installations in the region. Since those installations can achieve a TSS concentration of 20,000 in the digester, that will be used as a target in the design. Assume that an oxygen transfer device with a transfer efficiency of 1.2 kg 0_-/(kW hr) will be used.

a. What SRT might be appropriate for this application?

Figure 12.11 indicates that 40% stabilization can be achieved with a temperature-SRT product of 600°C-days. Since the lowest expected temperature is 10°C, the SRT should be 60 days.

b. What volume should the digester have?

The digester volume can be determined with Eq. 12.17. To use that equation the influent VSS and FSS concentrations must be known. Since the waste solids are 75% volatile:

Therefore,

Recognition of the fact that 1.0 mg/L = 1.0 g/m1 and substitution of these values into Eq. 12.17 gives:

c. What is the oxygen requirement?

The oxygen requirement can be estimated with Eq. 12.18 after an appropriate value has been chosen for i<lxMv Provided that pH control is practiced, nitrification will occur even in the winter because of the long SRT. Consequently, from Table 12.1, the appropriate value for i<,wi.v is 1.98 mg O ../mg VSS destroyed. Consequently, the oxygen requirement is:

d. What is the power requirement for oxygen transfer?

Assuming the use of an oxygen transfer device with a transfer efficiency of 1.2 kg O.Z(kW-hr), the required power input is:

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